Apparatus for producing artificial crystals



July 31, 1951 G. E. MERRITT APPARATUS FOR PRODUCING ARTIFICIAL CRYSTALS Filed Jan. 30, 1947 0rye M x? TTOHNEY! Patented July 31, 1951 APPARATUS FOR PRODUCING ARTIFICIAL CRYSTALS George E. Merritt, Elmira, Heights, N. Y., assignor to Bendix Aviation Corporation, a corporation of Delaware Application January 30, 1947, Serial No. 725,183

2 Claims. 1

"The present invention relates to a method and apparatus for producing artificial crystals and more particularly for growing crystals of piezoelectric material from a super-saturated solu tion thereof.

The production of artificial piezo-electric crystals by present commercial methods is a very slow and expensive process involving the gradual lowering of the temperature of the manufacturing space and liquor baths therein to produce a batch of crystals, after which new baths must be prepared, the space and baths again stabilized at the high temperature and the process repeated.

In such batch methods of growing crystals the size of the crystals is necessarily limited by the size and temperature range of the baths, and the growth of the crystals must be controlled mainly by the use of so-called buffers such as iron or titanium salts, and by the close control of the acidity of the baths.

It is an object of the present invention to provide a novel continuous process for the production of piezo-electric crystals.

It is another object to provide such a process in which the manufacturing space is maintained at a constant temperature.

It is another object to provide such a process in which the accretion of the crystal is brought about by continuously circulating the mother liquor past a seed crystal while in a super-saturated condition.

It is a further object to provide apparatus for eifectively and conveniently carrying out the novel process.

It is another object to provide such an apparatus in which the supply of crystal-forming salt may be replenished without interrupting the process.

It is another object to provide such an apparatus in which the growth of the crystal may be controlled by adjusting the speed of flow and the temperature range of the circulating liquor.

It is another object to provide such an apparatus including variable-speed circulating means which is arranged to produce a steady flow of the liquor past the crystal without surges or vibration.

Further objects and advantages will .be apparent from the following description taken in connection with the accompanying drawing in which:

Fig. l is a diagrammatic representation of an apparatus adapted to carry out the novel process;

and

Fig. 2 is a similar view of such apparatus rearranged to provide for flow of the crystal-iorm ing liquor in the opposite direction.

In Fig. 1 of the drawing there is illustrated a crystal-growing container I in which a seed crystal 2 is supported by means of a bracket 3 suspended from the top of the container. A storage chamber 4 having a supply of crystal-forming material 5 therein is connected to the crystalgrowing container I by means of a conduit 6 in the form of a syphon. A settling chamber or vessel I is located adjacent storage chamber 4 and may be formed as a unit therewith. The settling chamber is connected to the storage chamber by a syphon-conduit -8 in which is located a pump 10 of any suitable type arranged to transfer liquor from the storage chamber to the settling cham-- her. It will be understood that the pump is preferably of a rotary type the speed of which may be readily varied, and the operation of which creates a minimum of turbulence.

A conduit 5 including a syphon portion II, is arranged to connect the settling chamber 1 with the bottom of the crystal-growing chamber I,

' whereby when the pump p is operated, the liquor is caused to circulate continuously and to drift slowly upward through the growing chamber I past the crystal 2.

In order to fill the syphons 6 and II, taps I2 and I3 are taken off from the top portions thereof respectively and extend into cylindrical chambers I l and I5 to which aspirator tubes It, Il are connected.

According to the present invention, the circulating liquor in the growing chamber I is maintained at a substantially constant temperature, but the liquor that is withdrawn through the conduit '5 is warmed slightly by suitable means such as a heater coil indicated diagrammatically at I8, so that when the liquid reaches the supply chamber 4, it is capable of taking up an additional supply of the crystalline material 5. On the other hand, as the liquor traverses the return conduit 9 from the settling chamber 7 to the growing chamber I, it is cooled by suitable means such as cooling fins I9, the effect of which may be varied by controlling convective air currents therethrough. The efiect of this cooling is to render the liquor passing through the conduit super-saturated so that when it passes up through the growing chamber I the excess solute is deposited on the crystal 2.

Means indicated in the form of thermometers 2| and 22 are provided for showing the rise in temperature caused by the heating element I8,

and the lowering of the temperature caused by the cooling elements I9 respectively.

A heating element 23 is arranged to supply a small amount of heat to the settling chamber 1 when required to prevent formation of crystals therein, and similarly a heater element 24 located at the bottom of the growing chamber 1 is provided for introducing a small amount of heat to prevent the premature deposition of crystals at this point.

In carrying out the process in the preferred manner, the various chambers are initially filled as indicated with a saturated solution of the crystal-forming material such as ammonium dihydrogen phosphate or Rochelle salts, and the eX- posed surfaces of the solution are preferably covered with a layer of mineral oil such as indicated at 25 in order to prevent evaporation. Aspiration is then applied to the tubes l6, l1 until the liquor fills the syphons 3 and H and overflows into the chambers I4 and 15 up to the level of the liquor in the growing and storage chambers. The pump 1) is then started to cause a slow circulation of the liquor, and the energization of the heating element 18 adjusted to secure a suitable slight increase in temperature of the liquid passing from the growing chamber 1 to the storage chamber 4. The cooling elements I8 are rendered effective by means of air circulation or otherwise to reduce the temperature of the liquid passing therethrough by a substantially equal differential, and the system is permitted to oper ate steadily until the proper temperature conditions are stabilized.

A seed crystal 2 is then introduced into the growing chamber l, and a slight amount of distilled water added in order to temporarily reduce the concentration of the crystal-growing salt blow saturation. By this means any parasitic crystals on the surfaces of the seed crystal are dissolved, and a suitable surface provided on the seed crystal for the initiation of the growing process. After the circulation of the liquid has proceeded for a short time, sufficient crystalline material is taken up from the supply 5 to render the incoming liquid super-saturated as it enters the growing chamber I and the seed crystal 2 begins its growth. The process of growth is thereafter continuous, the material abstracted in the growing chamber being continuously replenished from the supply 5. The rate of circulation of the liquor as controlled by the pump p is so coordinated with the degree of supersaturation caused by the temperature differentials in the circulating system as to secure uniform growth of the crystal 2 in both directions. If the circulation is accelerated, the growth of the crystal will be mainl in the down-stream direction, whereas im the circulation is retarded, the growth will be increased in the rip-stream direction.

If during the operation of the device an air bubble should accumulate at the top of one of the syphons 6 or H, it may readily be removed by lowering the corresponding chamber ill or 15 so as to cause the liquor to flow up into the taps 12 or E3.

Inasmuch as the chambers are all open to atmospheric pressure, it will be seen that no surging or vibration due to the operation of the pump p can be transmitted to the liquor which leaves the settling chamber 1. A quiet and steady flow of the liquid is accordingly secured which prevents premature deposition of crystals, from the super-saturated liquor as it returns to the growing chamber i.

In the embodiment of the invention illustrated in Fig. 2, the parts of the apparatus are similar to those of the first embodiment and are similarly numbered, but the location of the heating and cooling means is reversed in order to secure flow of the liquor down past the seed crystal 2 in the growing container I instead of the upward flow produced by the apparatus in Fig. l. The method of operation of the apparatus is otherwise the same as that first described, and the growth of the crystal is brought about in the same manner.

It will be understood that the circulation of the liquor in either of the embodiments illustrated can be brought about merely by the thermosyphon effect of the heating of the conduit leading from the crystal-growing chamber and the cooling of the conduit leading into said chamber, in conjunction with the increase in density of the solution when it is replenished, and its decrease in density as it gives up crystalline material in the growing container. This of course facilitates securing the desired slow steady flow of the liquor without turbulence, and provides automatic regulation to some extent by virtue of the fact that an increase in the temperature differentials automatically increases the speed of circulation. In practice it has been found to be quite feasible to secure satisfactory growth of crystals by controlling the circulation of the liquid entirely by means of the temperature and density differentials, the pump being omitted. It has been found desirable, however, to be able to regulate the flow of liquor and the temperature differentials independently, and for this purpose the pump 10 is advantageous, although it is normally called upon to maintain only a very minute difference in level between the chambers 4 and 9.

Although certain steps in the continuous process disclosed have been described, and one form of apparatus illustrated, it will be understood that certain of the steps of the process may be altered or omitted, and other forms of apparatus may be used without departing from the spirit of the invention as defined in the claims appended hereto.

What is claimed is:

1. A11 apparatus for growing crystals of piezoelectric material including'a container for mother liquor, means for supporting a seed crystal therein, a vessel containing a storage supply of solute, said vessel comprising a solute storage chamber and a settling chamber, a conduit leading from the container to the solute storage chamber of the supply vessel and extending into said chamber to a point below the normal liquid level therein, means for warming the solution passing through said conduit, a return conduit leading from a point below the normal liquid level in the settling chamber or" the supply vessel to the crystal container, means for cooling the solution as it returns to the crystal container, and means for inducing a steady drift of the solution through said apparatus without vibration or turbulence in said crystal container comprising pumping means supported above the solute storage chamber independently of the conduits without any mechanical contact with either conduit, for maintaining the level of the solution in the settling chamber slightly above the level of solution in the supply chamber, said pumping means being of the type which delivers the material pumped in a continuous flow, substantially free from pulsations.

2. An apparatus as set forth in claim Lin 5 which the conduit from the crystal container to the supply chamber is in the form of a syphon extending nearly to the bottom of the supply chamber, and the return conduit from the settling chamber to said crystal container is in the form of a syphon extending nearly to the bottom of the settling chamber. 7

GEORGE E. MERRITT.

REFERENCES CITED 6 UNITED STATES PATENTS Number Name Date 1,353,571 I Dreibrodt Sept. 21, 1920 2,459,869 Christensen Jan. 25, 1949 FOREIGN PATENTS Number Country Date 482,096 Great Britain Mar. 23, 1938 

